Regulatory air-pollution limits for diesel engines have caused some manufacturers to adopt selective catalytic reduction (SCR) technology for reducing nitrogen oxides (NOx) in engine exhaust. The SCR process introduces an NOx reducing compound (e.g., a urea-water solution) into the hot exhaust gas, which chemically reduces NOx into non-pollutant compounds in conjunction with a catalyst. However, during typical operation of an engine, the conditions for the SCR process are not optimal (e.g., exhaust is too hot or cold) and either too much, or too little, urea solution is introduced into the exhaust. When urea solution is introduced that does not undergo the SCR process (e.g., too much urea solution for the exhaust temperature), urea crystals will accumulate within the exhaust system, both on the interior surface of the exhaust pipe and at the port that introduces the solution into the exhaust. Build up of urea crystals in the exhaust system detrimentally affects the performance of the exhaust system, and also is indicative of inefficiency in the SCR process: urea crystals represent both wasted urea solution and reduced SCR efficiency.
During injection of urea into the diesel exhaust stream during SCR there is a significant amount of the total urea injected that contacts the walls of the exhaust pipe and becomes a liquid wall film. While this process occurs, the urea that is wetting the pipe walls does not reach the catalyst for its intended use and the intended quantity of reactant is not available in the catalyst.
In order to provide the minimum required urea amount, during normal engine operation, the urea injector cycles on and off. While in operation, the aqueous urea solution is injected as a stream of small droplets. Effective management of these droplets requires injector geometries specifically intended to transport the droplets away from the injector and into the main pipe flow with minimal wall wetting.
Known methods for reducing wall wetting during urea injection include utilizing a urea “doser port” having a urea injector and a chamber that opens at the side of the main exhaust pipe, such as urea dosers manufactured by Bosch. A diagrammatic illustration of an exemplary prior art doser port 704 is illustrated in FIG. 7, wherein the doser port 704 comprises a urea doser 708 and a chamber 710 shaped to provide passage for injected urea 712 into an exhaust pipe 720. However, such doser designs do not eliminate the wall-wetting effect, particularly because exhaust flow 724 and 726 sometimes works against the doser port 704 by pushing the injected urea 712 into the walls of the doser port 704 (thus promoting wall wetting), as illustrated by exhaust flow 726.
For example, the main exhaust gas flow 724 and 726 cause the gas in the doser port 704 to move at high velocities. In turn, these high gas velocities push the urea droplets into the walls that define the chamber 710 (see, e.g., exhaust gas flow 726). A droplet that sticks to the wall forms a liquid film. Under certain temperature conditions, the wall film will then form undesirable urea crystals. Additionally, this configuration causes problems with the last droplets injected in an injection cycle. In that regard, at the end of an injection cycle, the last droplets are injected with decreasing velocity. As the injection velocity approaches zero, it is increasingly difficult for the droplets to reach the main exhaust pipe 720. Therefore, the last drops typically fall into the chamber 710 wall and form a film.
So as to at least reduce the above-described inefficiencies with regard to urea injection during an SCR process, a system not heretofore developed is needed, among others, to facilitate injection of liquid urea solution into an exhaust system while reducing wall-wetting effects.